Field of the Invention
The present invention relates to an image pickup apparatus that changes gain of an amplifier of an image pickup device, a control method therefore, and a storage medium that stores a control program therefor.
Description of the Related Art
In recent years, an image pickup apparatus, such as a digital still camera or a video camera, is miniaturized, and the number of pixels of an image pickup device like a CMOS image sensor thereof increases, which makes a cell (pixel) size be fine. Then, the finer the cell size is, the lower the sensitivity of an image pickup device is unescapably. Furthermore, high frame rate is set for shooting a video image in an image pickup apparatus in order to obtain a smooth video image. For example, a video image is obtained by controlling reading of signals from an image pickup device at the frame rate of 60 frames per second.
Accordingly, if a video image is taken in low illuminance, drop of an S/N ratio due to the degradation of the sensitivity of an image pickup device will deteriorate image quality as a result.
In order to avoid such a fault, there is a known image pickup apparatus that reduces the total noise amount by changing a control gain value of a gain amplifier (AMP) for each column according to illuminance of a subject (see Japanese Laid-Open Patent Publication (Kokai) No. 2009-49981 (JP 2009-49981A)).
Incidentally, offset levels of the gain amplifiers of the respective columns with which the image pickup device is provided are different with one another. Accordingly, the image pickup apparatus described in the above-mentioned publication may cause what is called a longitudinal line flaw or a black level deviation in images of several fields through several tens of fields immediately after the change of the control gain values due to the difference among the offset levels of the gain amplifiers of the respective columns. Then, the longitudinal line flaw and the black level deviation deteriorate the image quality.
In order to cope with the difference among the offset levels, there is a known technique that enlarges a coefficient (time constant) of a correction filter, which is used in a vertical optical black (VOB) correction process for subtracting a projection output of a VOB area from an output of an effective area, immediately after changing the control gain values of the gain amplifiers of the respective columns. Then, this technique minimizes the black level deviation and the longitudinal line flaw at the time of changing the driving mode of the image pickup device by enlarging the coefficient of the correction filter to shorten filter convergence time.
Incidentally, since the convergence time of the correction filter is inversely proportional to the number of lines in the vertical optical black (VOB) area with which the image pickup device is provided, it needs to increase the number of lines in the VOB area read from the image pickup device as many as possible.
On the other hand, in order to obtain a smooth image in the case of taking a video image, it is necessary to output the video signal at the frame rate of 60 frames per second as mentioned above. As a result of this, the reading time for one frame is determined (16.67 msec per frame).
Furthermore, the number of lines read from the image pickup device is the total of the number of the reading lines in the VOB area and the number of the reading lines in the valid pixel area that is used for a video signal and electronic image stabilization. In order to output the video signal of one frame at a specified frame rate, it is necessary to restrict the number of lines through which the signals are read from the image pickup device. Accordingly, if the number of the reading lines in the VOB area increases, the number of the reading lines in the valid pixel area used for the video signal and the electronic image stabilization will decrease relatively.
Reduction of the number of lines used for the video signal in this way will degrade vertical resolution. Furthermore, when a surplus effective pixel area that is defined by excepting the video signal area from the valid pixel area decreases, an image stabilization area in which a segmenting position varies will be narrow, which deteriorates an image stabilization performance.